Range Description

Range Description

Overall Distribution:The Franciscana dolphin inhabits shallow coastal and estuarine waters of tropical and temperate regions of the western South Atlantic Ocean. Its distribution extends from Itaúnas (18°25?S), Espírito Santo State in southeastern Brazil (Siciliano, 1994), to Golfo Nuevo (42°35?S), Chubut Province, Argentina (Crespo et al. 1998). However, the species is not distributed continuously throughout its range. Surveys (including beach surveys, visits to museum collections, interviews with local people) indicate that franciscanas are extremely rare or absent in two areas of the northern parts of their range between Macaé (southern Rio de Janeiro State) and Ubatuba (northern São Paulo State) and in southern Espírito Santo State (Azevedo et al. 2002, Siciliano et al. 2002, Secchi et al. 2003). The reasons for these gaps are unclear, but because the species prefers shallow, turbid waters (Pinedo et al. 1989, Brownell 1989), water transparency and depth may be among the factors responsible (Siciliano et al. 2002).

Distribution and Population StructureThis listing hinges in large part on the evidence for a separate Franciscana subpopulation or stock centered in Rio Grande do Sul (Brazil) and Uruguay. Therefore, much of the discussion here focuses on that evidence.

Morphological and molecular data strongly support the existence of two main subpopulations. Multivariate analyses of morphometric data revealed two geographical forms: a smaller form in the northern part of the species? range (north of 27°S) and a larger form in the coastal waters of southern Brazil, Uruguay, and Argentina (south of 32°S) (Pinedo 1991). Analyses of a highly variable region (d-loop) of mitochondrial DNA (mtDNA) also supported these two geographical forms (Secchi et al. 1998). Furthermore, Lázaro (2000, 2001) and Hamilton et al. (2000) compared the mtDNA of franciscanas from Uruguay and Argentina with those published by Secchi et al. (1998) and found support for the existence of a large southern subpopulation (composed of animals from Rio Grande do Sul, Uruguay, and Argentina) that is clearly differentiated from animals in the waters of Rio de Janeiro (northern subpopulation). These studies revealed fixed genetic differences between the subpopulations that suggest essentially no effective genetic exchange (see Secchi et al. 1998, Hamilton et al. 2000, Lázaro 2001). Although the genetic evidence is not very strong for splitting the southern subpopulation further, a pairwise analysis of haplotype distances between different geographical locations showed increasing differentiation in haplotype frequencies with increasing geographical distance, following an isolation-by-distance pattern (Lázaro 2001). Moreover, a recent analysis showed that haplotype frequencies of samples from Claromecó (Argentina) were significantly different from frequencies for the rest of the southern subpopulation (Lázaro 2001). This finding, when considered together with other biological differences between animals from Argentina and Rio Grande do Sul/Uruguay (Secchi et al. 2003a), is consistent with the idea that the southern subpopulation should be divided into different stocks for management.

Danilewicz et al. (2000) compared estimates of age at sexual maturity (ASM) of females incidentally caught in gillnets in Rio Grande do Sul with estimates from Uruguay (adapting data from Kasuya and Brownell 1979) and northern Argentina (Corcuera 1996). Using the method of DeMaster (1984), the resulting ASM was 3.7 years (95% CI = 3.0 4.4) in Rio Grande do Sul, 2.8 years (95% CI = 2.5 3.1) in Uruguay, and 4.5 years in Argentina. The ASM of females from Rio Grande do Sul was significantly higher than that of females from Uruguay (t = 2.3; P<0.01) but lower than that of females from Argentina.

The distribution pattern of gastrointestinal helminths was similar for franciscanas sampled in Rio Grande do Sul and Uruguay (Andrade et al. 1997). The animals were infected lightly by the acanthocephalan worm Polymorphus cetaceum, while all franciscanas sampled in Argentina exhibited high levels of infection. The dominant parasites were also different between animals from Rio Grande do Sul and Argentina. The trematode Hadwenius pontoporiae was present in 83% of the dolphins sampled in Rio Grande do Sul and P. cetaceum was found in 63% of the franciscanas from Argentina. Aznar et al. (1995) compared the helminthofauna of franciscanas in Uruguay and Argentina and suggested some differences in dominance and infection levels by P. cetaceum, Anisakis simplex, and H. pontoporiae. The studies of Aznar et al. (1994, 1995) and Andrade et al. (1997) showed that the parasite infection levels in franciscanas from Rio Grande do Sul and Uruguay were similar to each other but considerably different from those sampled in Argentina. These findings suggest the existence of two ecologically distinct stocks: one in Rio Grande do Sul and Uruguay and the other in Argentine waters.

By applying the hierarchical classification scheme of Dizon et al. (1992) for determining the likelihood of stocks being evolutionarily significant units, Secchi et al. 2003a) proposed (based on the above data as well as other information on distribution, morphometrics, and life history parameters) that the southern and northern subpopulations of the Franciscana each be divided into two management stocks. The range limits for each stock were defined as provisional Franciscana Management Areas (FMAs), as follows: FMA I coastal waters of Espírito Santo and Rio de Janeiro states, Brazil (note that with increased survey effort, confirmation of the hiatus in the Espírito Santo State will require further division of this FMA); FMA II São Paulo, Paraná and Santa Catarina states, Brazil; FMA III coastal waters of Rio Grande do Sul State, southern Brazil and Uruguay; and FMA IV coastal waters of Argentina, including the provinces of Buenos Aires, Rio Negro and Chubut.